Optical devices such as wavelength shifters and wavelength converters have been used to "change the wavelength" of an optical data signal. This change in the wavelength is more properly exemplified as a transfer of data or information from a carrier signal at a first wavelength to a different carrier signal at a second wavelength.
Wavelength shifters or converters are applicable to lightwave transmission networks as such networks employ wavelength division multiplexing and wavelength routing of optical signals. These devices help to overcome the capacity limitations of such networks by rearranging (wavelength channel interchange) and reallocating the optical wavelength channels for efficient use of the limited optical bandwidth of the network.
Optical wavelength conversion has been demonstrated by using a traveling wave semiconductor optical amplifier which performs the conversion on intensity modulated optical signals by employing either four-wave mixing or the gain saturation effect. These devices suffer from a common shortcoming. The signal extinction ratio on the converted signal and/or the conversion efficiency are neither very high nor attractive over a wide range of wavelengths without the expenditure of relatively high input signal power.
Semiconductor optical amplifiers have been used more recently in both Mach-Zehnder and Michelson interferometers to provide wavelength conversion. Amplifiers were arranged in both arms of each interferometer. These configurations exploit the phase shift caused by the refractive index variation associated with gain saturation in the optical amplifiers. As such, the interferometers transfer attendant phase modulation into an amplitude (intensity) modulated signal. This technique apparently operates at low power while improving the signal quality of the converted signal with respect to extinction ratio and chirp.